This possibly is a solution to the final question: http://ac.els-cdn.com/0304397585900349/1-s2.0-0304397585900349-main.pdf?_tid=0e733f14-8779-11e7-9492-00000aab0f26&acdnat=1503434146_79c9bc9bf95106214e663bd9b5e7dcb2
Actully, it is not only relating to Union of languages,but homomorphism and intersection. And there are several other solutions to the question in the reviewing part of the article of the URL.
Dyck language is defined as $S\to SS|[S]|\epsilon$.
Minimal linear grammars are defined as the production rules in the following form:
$S\to uSv|\omega, \omega, u,v\in \Sigma^*$ where $\Sigma^*$ is the Kleene closure.
But according to Bjørn Kjos-Hanssen's post, if I have not misunderstood, it is impossible to have low complexity languages to form higher complexity languages by finite or infinite union